.box {
    width: 808px;
    height: 498px;
    background: url(./课表.png) no-repeat center center /100%;
    margin: 2em auto;
    position: relative;
}

.box::after {
    filter: blur(10px);
    z-index: -1;
    content: "";
    position: absolute;
    left: 20px;
    right: 0;
    top: 20px;
    width: 100%;
    height: 100%;
    background: conic-gradient(
            #ffff00,
            #2e6da4,
            #8a6d3b,
            #faebcc,
            #761c19
    );
}